1. Field of the Invention
The present invention generally relates to a motor-driven power steering control apparatus designed for controlling application of a steering assist force or torque generated by an electric motor to a steering system of a motor vehicle for assisting an operator or driver in his or her steering manipulation. More particularly, the present invention is concerned with a control which is carried out upon starting and stopping of driving operation of the electric motor mentioned above.
2. Description of Related Art
The conventional motor-driven power steering control apparatus for controlling application of an steering assist force or torque in driving a motor vehicle as generally known in the art is so designed as to detect a steering torque of a steering system of the motor vehicle for generating a steering assist torque by means of an electric motor on the basis of a detection signal indicative of the active steering torque to thereby reduce or mitigate the steering effort of the operator or driver.
By way of example, in the hitherto known or conventional motor-driven power steering control apparatus, a steering wheel or handle is operatively coupled to a steering column so that a steering torque applied to the steering wheel by the driver is transmitted to the steering column. A torque sensor is provided in association with the steering column and is so designed as to supply an electric signal indicative of the detected steering torque to a control processor circuit. The control processor circuit is constituted by a CPU (Central Processing Unit), a ROM (Read-Only Memory), a RAM (Random Access Memory), I/O (Input/Output) circuits, a PWM (Pulse-Width Modulation) circuit and an A/D (Analogue-to-Digital) converter circuit.
The control processor circuit is designed to make decision as to application of the steering assist torque as well as stoppage thereof by making use of an ignition signal indicating turn-on/off of an ignition switch and an engine rotation speed signal (rpm signal) outputted from an engine rotation speed sensor. Further, when the steering assist torque is to be applied, the control processor circuit starts driving operation of the electric motor electrically connected to a battery by way of a bridge circuit on the basis of a vehicle speed signal outputted from a vehicle speed sensor, a steering torque signal outputted from the torque sensor and a current detection signal outputted from a current detecting circuit which is designed to detect the electric current flowing through the electric motor, whereby a steering assist torque is applied to the steering system. For particulars, reference may have to be made to Japanese Patent Application Laid-Open Publication No. 290764/1997 (JP-A-9-390764), p. 9, FIG. 1.
The conventional motor-driven power steering control apparatus of the structure mentioned above is disadvantageous in the respect that when application of the steering assist torque is immediately stopped in response to opening or turn-off of the ignition switch of the motor vehicle, a counter steering torque (steering force or torque in the reverse direction) may be put into effect due to restoration effort of the tires from the twisted state brought about by the steering assist torque applied during the steering manipulation, as a result of which the active steering torque changes or varies rapidly, incurring steep increase of the steering force or torque required for the driver. Under the circumstances, such measures are generally adopted that application of the steering assist torque is not interrupted or stopped until the active steering torque has become smaller than a predetermined value inclusive thereof after issuance of the command for stopping application of the steering assist torque, with a view to preventing or suppressing occurrence of variation of the active steering torque. However, in case the driver steers counter or quickly in the course of steering manipulation, application of the steering assist torque is instantaneously stopped at a time point when the steering torque transiently becomes neutral (i.e., upon passing by a neutral point), the steering force or effort required for the driver will steeply increase to make uncomfortable the steering operation, giving rise to a problem. Besides, the conventional motor-driven power steering control apparatus suffers a problem that the application of the steering assist torque may erroneously be determined under the disturbance of noise.
In the light of the state of the art described above, it is an object of the present invention to provide a motor-driven power steering control apparatus which can properly determine a timing or time point at which application of the steering assist torque is started or stopped.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a motor-driven power steering control apparatus for a motor vehicle which apparatus includes an electric motor for applying a steering assist torque to a steering system, a torque sensor for detecting a steering torque of the steering system of the motor vehicle, a motor driving unit for operating the electric motor in dependence on a detected value of the steering torque, a motor operation command unit for commanding starting and stopping of driving operation of the electric motor, and a motor operation decision unit for deciding starting and stopping of the driving operation of the electric motor on the basis of a command issued by the motor operation command unit. The motor operation decision unit is so arranged that when stopping of the driving operation of the electric motor is commanded by the motor operation command unit, the motor operation decision unit stops the driving operation of the electric motor while commanding the motor driving unit to stop application of the steering assist torque, provided that a state in which a detected value of the steering torque is not greater than a value predetermined for stopping the driving operation of the electric motor has continued for a time period predetermined for the stoppage.
By virtue of the arrangement described above, there can be realized the motor-driven power steering control apparatus which is capable of determining the steering status accurately, whereby the application of the steering assist torque can be stopped by interrupting the motor current at an appropriate timing while suppressing the torque variation due to kickback and ensuring comfortableness for the steering manipulation.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.